Vacuum coating



May 3, 1960 c. A. BAER ETAL. 2,935,589

VACUUM COATING Filed May 16, 1957 mi n. w wmw m A T.. p W O r M m m 3 mm 2 F 2 w 4 J I 8 l a, 2 J 7 United States Patent VACUUM COATING Charles A'. Baer, Needham, and Robert W. Steeves, Nahant, Mass, assignors, by mesne assignments, to National Steel Corporation, Pittsburgh, Pa., a corporation of Delaware Application May 16, 1957, Serial No. 659,593 1 Claim. (Cl. 219-1049) This invention relates to vacuum coating and more particularly to improved apparatus for evaporating metals for deposition on a substrate contained within a vacuum chamber. i

A principal object of the present invention is to provide an improved heating means for evaporating metals.

Another object of the invention is to provide a single turn induction heater.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure and the. scope of the application of which will be indicated in the claim.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. l is a view of one embodiment of the invention;

Fig. 2 is a diagrammatic view of one embodiment of the invention;

Fig. 3 is a diagrammatic view of another embodiment of the invention, and;

Fig. 4 is a simplified circuit diagram.

The use of multi-turn induction heating coils for evapcrating metals in a vacuum for coating purposes is well known. For example, US. Patents 2,584,660 and 2,643,- 201 and many others illustrate such induction coils. The use of a single multi-turn induction coil for heating a crucible containing the metal to be evaporated was satisfactory where the substrate to be coated was not too Wide. As the demand for vacuum coated materials increased, it became necessary to coat wider substrates. Obviously, a single crucible was not sufficient to provide a uniform coating across the wider substrates. As a result there was employed a plurality of crucibles heated by a plurality of multi-turn, induction coils. However, these have not proven to be entirely satisfactory for a number of reasons. First, the spacing between the coil turns and the diameter of coil turns are critical. Second, these rnulti-turn coils are not rigid and there fore are deformed very readily. Third, it is difiicult to position the crucibles within the coils so that each crucible being heated has the same characteristics such as equal crucible temperature and hence equal metal evap-.

oration rates. Fourth, the voltage drop across the individual coils varies as a function of direct current resistance and inductance of the coil. It is obvious that if the coil turns are not in proper alignment, then the positioning of the crucibles within each of the multi-turn coils being used so that each crucible will attain an equal temperature is very diflicult, if not impossible. It should also be pointed out that if equal crucible temperatures are not achieved, then the metal evaporation rates will vary and the attainment of a uniform coating on the substrate is difiicult. One further disadvantage to the a use of multi-turn induction coils is the difficulty in removing the crucibles therefrom after they have been used.

The present invention is directed to an improved apparatus for coating a substrate by evaporating a metal or the like in a vacuum chamber and depositingthe metal on the substrate. The present apparatus eliminates the disadvantages of the multi-turn induction coil mentioned above and provides instead a source which can coat wide substrates uniformly at high speed. In one embodiment of the present invention there is provided a vacuum chamber, a high frequency generator outside of the chamber, a crucible for holding the metal to be evaporated inside the chamber, a single turn induction coil surrounding the crucible for heating the metal to its vaporization temperature, a bank of condensers positioned within the vacuum chamber and connected to the coil by short bus-bars having low electrical resistance to minimize losses due to the heavy currents flowing in the tank circuit comprising the coil and the condenser bank, and a coaxial lead or leads extending into the vacuum chamber for connecting the high frequency generator with the tank circuit.

In one embodiment of the invention the single turn induction coil com-prises two half coils connected in series, the two half coils surrounding the crucible to form a full split single turn coil around thecrucible. When a plurality of crucibles are utilized, then there is employed a plurality ofhalf coils connected in series, pairs of said half coils surrounding individual crucibles to form a full split single turn coil around each crucible.

In another embodiment of the invention there is utilized an elongated crucible which is provided with a single turn coil conforming to the shape of the crucible. The series assembly of single turn coils of the present in vention has many advantages over the use of multi-tu-r'ii' coils. For instance, since each coil has only one turn and the current through the entire tank circuit is the same, then electrical uniformity is as close as is possible. Also the field in each coil is such that exact positioning of the crucibles is less critical. Additionally, the single turn coil assembly is more rigid than the multi-turn coils.

Referring now to Fig. 1, there is illustrated a single half coil 2. The half coil 2 is constructed of a conducting material, preferably copper. In Fig. l the half coil is shown as being formed from a fiat, solid bar of copper. However, the half coil can be constructed of suitable copper tubing, rods or the like. The half coils are preferably providedwith suitable cooling means 3. One manner of cooling the half coils is to attach thereto copper tubing through which a coolant such as water can be circulated. Also the half coils themselves can be constructed so as to be provided with a suitable coolant passage therein.

Fig. 2 illustrates a plurality of 'half coils 2, the half coils 2 being joined so that they form a series arrangement. Pairs of half coils 2 surround individual crucibles 4 to form a full split single turn coil around each crucible. The half coils 2 on each side of the crucibles 4 are electrically insulated from each other by means of a suitable high temperature insulating material 6. As illustrated in Fig. 1 the half coils are joined by means of a conducting material which is of considerably larger area and volume than the half coils. This provides to the half coil system (a) rigidity, (b) proper spacing and (0) low voltage drop between the half coils.

Fig. 3 illustrates the use of an elongated crucible 9 surrounded by a single turn inductor 8. The two ends of the inductor are electrically insulated from each other by means of a suitable high temperature insulating material 10. The inductor is preferably constructed of cop- 3 per and provided with suitable cooling means such as is illustrated in Fig. l.

Fig. 4 illustrates in simplified form a circuti of the instant invention. Within the vacuum coating chamber 12, there is located one or more single turn inductors 14 in series which can be of the kind illustrated in theabove figures. Also positioned within the vacuum coating chamber 12 is a bank of condensers 16, connected in parallel with the inductors for tuning the tank circuit to resonance at the frequency of the generator 28. The condenser bank 16 is connected to the inductors by short bus-bars of low electrical resistance such as copper to minimize losses due to the heavy circulating currents in the tank circuit comprising the inductors and the condenser bank. When the inductors are in operation, there is a high circulating current in the circuit comprising the inductors and the condenser bank. If the condenser bank 16 is located outside the vacuum coating chamber 12, then there is a very high cur-rent passing through the feed-through or coaxial leads into the vacuum chamber. These leads are not sufficient to handle such large currents and as a result high voltage drops occur which impair the source efiiciency. Thus in order to eliminate the high circulating current in the feed-through and to.

achieve uniform heating, it is necessary to locate the condenser bank in the vacuum coating chamber 12. The condenser bank connection to the inductors is as short as possible and preferably made by short bus-bars of copper.

Coaxial lead 22, connected to the inductors 14 and condenser bank 16, pass through a suitable vacuum seal 24. The coaxial lead is connected through a suitable transformer 30 to a generator or source of high frequency electrical energy 28. The generator 28, adapted to energize the inductors 14, is preferably an alternating current generator supplying current having a frequency of about 3 to 500 k-ilocycles per second. A second condenser bank 26 is utilized to provide fine tuning of the tank circuit.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

An apparatus for coating a substrate by evaporating a metal or the like in a vacuum chamber and depositing the metal on the substrate, the improvement which comprises a source of high frequency current outside of the vacuum chamber, at least one crucible for holding the metal to be evaporated inside the vacuum chamber, a single turn induction coil surrounding the crucible for heating the metal to its vaporization temperature, a bank of condensers positioned within the vacuum chamber and connected to the coil by short bus-bars having a low electrical resistance to minimize losses due to the heavy currents flowing in the tank circuit comprising the coil and the condenser bank, and a coaxial lead extending into the vacuum chamber 'for connecting the source of high frequency current with the tank circuit, the tank circuit being tuned so that current flow in the tuned circuit is much greater than current flow through the coaxial lead.

References Cited in the tile of this patent UNITED STATES PATENTS 2,321,189 Dravneek June 8, 1943 2,402,508 Strickland June 18, 1946 2,472,370 Drugmand June 7, 1949 2,570,311 Bohnet et al. Oct. 9, 1951 2,643,201 Chadsey et al. June 23, 1953 2,676,235 Kennedy et al. Apr. 20, 1954 FOREIGN PATENTS 147,558 Switzerland Aug. 17, 1931 

